Two Firings, Two Different Jobs

Almost every pot you make will be fired at least twice, and the two firings are doing completely different work. The bisque firing turns soft, fragile greenware into a durable, porous object you can handle and glaze. The glaze firing then melts that glaze into a glassy skin that fuses permanently to the clay surface. Get the temperatures right and the whole thing works quietly in the background. Get them wrong and you will meet crazing, pinholing, shivering, or a glaze that simply slides off onto your kiln shelf.

The single most useful idea to hold on to is this: your clay body and your glaze have to agree on a temperature. Clay matures over a range, glaze melts over a range, and those ranges need to overlap.

Bisque Firing: Turning Clay into a Sponge

For most earthenware and mid-range stoneware bodies, a bisque firing of around 950–1000°C works well. Some potters push to 1000–1060°C for a denser, more robust bisque, particularly for sculptural work or pieces that need a lot of handling.

What you are aiming for is a body that has lost its chemically combined water, had its organic matter burned out, and sintered just enough to be strong — but which is still open enough to absorb water and glaze. Go too low (below roughly 900°C) and the pot stays chalky and friable, and glaze may flake off the surface. Go too high and the body begins to close up, so it will not take glaze evenly, and you can trigger pinholing and blistering in the glaze firing that follows.

Give the kiln time to breathe during bisque. A slow ramp up to around 600°C, with a good soak, allows carbon and sulphur to burn out cleanly rather than getting trapped in the clay.

Glaze Firing: Where the Surface Comes Alive

The glaze firing is where temperature does its most visible work. Glazes are formulated to melt within a specific band, and they are usually described by cone rather than by a single number:

• Low fire — roughly cone 06 to 04, about 990–1060°C
• Mid fire — roughly cone 4 to 7, about 1180–1240°C
• High fire — roughly cone 8 to 12, about 1250–1300°C

These are approximations, because cones measure heat work rather than raw temperature, and every kiln behaves a little differently.

Heat Work, Not Just Temperature

Heat work is the combination of how hot you go and how long you stay there. A kiln that touches 1240°C and immediately begins cooling has done less work than one that holds at 1240°C for thirty minutes. This is why a pyrometer reading and a cone can disagree, and why the cone is the more honest judge of what your pots actually experienced.

Two practical consequences follow. First, if your glazes are coming out underfired even though the controller says you hit the target, add a soak at the top of the firing — often 20 to 40 minutes — rather than raising the peak temperature. Second, when you change your firing schedule, you have effectively changed your glaze recipe, so expect results to shift.

Matching Clay Maturity to Glaze Melt

Every clay body has a maturation point: the temperature at which it reaches its intended strength, density and porosity. Firing below it leaves the body weak and absorbent; firing above it can cause bloating, warping and slumping.

• Earthenware matures around 1040–1150°C and stays porous even when fully fired
• Mid-range stoneware matures around 1200–1260°C, where it becomes strong and near-vitrified
• High-fire stoneware and porcelain mature around 1240–1300°C

The rule is straightforward: choose a glaze that reaches full melt at the temperature where your clay matures. A high-fire glaze on earthenware will never melt properly — it will sit dry, powdery and prone to leaching. A low-fire glaze on a stoneware body heated to 1240°C will boil, run and possibly take your pot's foot ring with it.

For functional ware, this matching matters beyond appearance. A well-matured body combined with a fully melted, durable glaze gives you a surface that is hygienic and food safe. A glaze that has not reached its melting point can remain soluble and release colouring oxides into food.

Defects That Come From a Mismatch

Most firing faults are a compatibility problem in disguise:

• Crazing — a fine web of cracks in the glaze, usually because the glaze is shrinking more than the body beneath it
• Shivering — glaze flaking away at rims and edges, the opposite mismatch
• Pinholing and blistering — often an over-fired body trapping gases, or a glaze firing too fast
• Running and shelf sticking — glaze fired beyond its intended cone
• Dunting — cooling cracks caused by stressing the body through a glaze firing it was never meant for

Testing, Recording, Repeating

Make test tiles from every clay body you use, glaze them with the glazes you intend to combine, and fire them alongside your work. Keep a log of peak temperature, soak time, ramp rates and cone results, and change only one variable at a time. Over a few firings you will build a reliable picture of what your kiln and your materials actually do — and that knowledge is worth far more than any chart.